1. Field of the Invention
The present invention relates in general to an inexpensive dispersion slope compensator able to equalize the energies of output optical signals.
2. Description of the Related Art
The development and use of optical fibers has greatly increased the ability of people around the world to communicate. Due to the swelling requirements of wide bandwidth, current trends in the designs of the optical fiber networks tend towards increasing volume, reliability and economy. With such requirements, a wavelength division multiplexing (WDM) transmission system is the best choice.
It has been noticed that too large a dispersion or a long-distance signal transmission easily causes signal interference in any a wave band. Furthermore, the extent of accumulated dispersions for each optical signal in a WDM transmission system is always different, so using a single dispersion compensation fiber to simultaneously compensate the accumulated dispersions for all the optical signals perfectly is impossible. Thus, various techniques of controlling the compensation for dispersion are being actively developed.
FIG. 1 depicts a dispersion slope compensator according to a first example of the prior art, wherein mixed optical signals of wavelengths .lambda..sub.1, .lambda..sub.2 propagating in an optical fiber 11 are transmitted to optical filters 15, 16 through a fiber coupler 12. The optical filters 15, 16 are used to select the optical signals. For example, the optical filter 15 only allows the optical signal of wavelength .lambda..sub.1 to propagate into a dispersion compensation fiber 17, while the other optical filter 16 only allows the optical signal of wavelength .lambda..sub.2 to propagate into another dispersion compensation fiber 18. The two dispersion compensation fibers 17, 18 are of different lengths so that the respective dispersions of the optical signals of wavelengths .lambda..sub.1 and .lambda..sub.2 are compensated perfectly. Then, the two compensated optical signals of wavelengths .lambda..sub.1 and .lambda..sub.2 are combined by another fiber coupler 19.
In the first example, three optical filters and dispersion compensation fibers will be used if three optical signals of different wavelengths .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 are mixed and simultaneously transmitted. By analogy, N optical filters and dispersion compensation fibers are used when N optical signals of different wavelengths .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N are mixed and simultaneously transmitted.
FIG. 2 depicts another dispersion slope compensator according to a second example of the prior art, which includes two arrayed waveguide gratings (AWGs) 21, 22 and a plurality of dispersion compensation fibers 23 connected therebetween. An optical fiber for propagating mixed optical signals of wavelengths .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N is connected to one of the inputs 1, 2, . . . , N of the arrayed waveguide grating 21. The arrayed waveguide grating 21 separates the mixed optical signals into the dispersion compensation fibers 23. The dispersion compensation fibers 23 are of different lengths so that the respective dispersions of the optical signals of wavelengths .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.N are compensated perfectly. Then, the compensated optical signals are combined by the other arrayed waveguide grating 22 and sent out from one of the outputs 1', 2', . . . , N' of the arrayed waveguide grating 22.
The disadvantages of the first and second examples mentioned above are now discussed. In the first example, the use of optical filters 15, 16 results in a great deal of energy loss. The more channels used, the more energy of the transmitted optical signals lost. The energy loss of the transmitted optical signals would be 6 dB for two channels and 9 dB for three channels.
Furthermore, both of the two examples mentioned above fail to equalize the energy of each channel. To solve this problem, erbium-doped fiber amplifiers (EDFAs) generally are provided after the dispersion compensation fibers for equalizing the strengths of each optical signals. However, EDFAs are very expensive, the use of which greatly increases the cost of building the WDM transmission system.